BHS collapsed this year with the loss of 11,000 jobs and leaving a £571m pensions deficit, prompting a public outcry and a vote by MPs to strip Sir Philip of his knighthood.

The Pension Regulator's chief executive Lesley Titcomb said: "Our decision to launch enforcement action is an important milestone in our work to attain redress for the thousands of members of BHS schemes who have been placed in this position through no fault of their own.

"Issuing warning notices at this time reflects the outcome of our investigations and that we are yet to receive a sufficiently credible and comprehensive offer in respect of the BHS schemes.

"We continue to pursue the best deal for members of the BHS pension schemes. If parties wish to approach us with settlement offers, that course remains open to them."

The regulator said the warning notices, each running to more than 300 pages, set out the evidence to support the use of its powers which can include demanding payment to the pension scheme or more flexible proposals.

Video:Sky News tries to talk to Sir Philip Green

Such powers have been used rarely, and never on the scale of that which is being applied to Sir Philip, whose high street empire includes brands such as Top Shop and Dorothy Perkins.

Those receiving the notices now have a set time to respond.

Once responses have been considered, the case can be passed on to a determinations panel to decide on the use of the regulator's powers.

Sir Philip Green said in a statement in response: "I have read the statement from the Pensions Regulator this evening and noted its contents.

"I have provided the Regulator with what I believe to be a credible and substantial proposal, with evidence and bank confirmation of cash availability, which would prevent the scheme from entering the Pension Protection Fund.

Video:BHS: Decline of a retail giant

"This is in order to achieve a better outcome for the BHS pensioners.

"I have also spoken to the chairman of the trustees who is supportive of the proposal on the basis that it provides members with better benefits than they would receive from the PPF.

"I believe the above statement confirms the statement of intent that I made in regard to the BHS pensioners."

The latest developments in the negotiations over BHS's pension deficit come more than four months after Sir Philip told MPs on two select committees that he would "sort" the issue.

He repeated that pledge in several letters to MPs and in a rare media interview last month.

Labour MP Frank Field, chair of the parliamentary Work and Pensions Committee and an arch-critic of the retail tycoon, welcomed the regulator's announcement.

He said: "We are not surprised that the Pensions Regulator has, like all the rest of us, lost patience with Sir Philip Green's excuses and empty promises."

Ian Wright, chairman of the Business Select Committee, which questioned Sir Philip, told Sky News: "He's meant to be the consummate deal maker and yet he's played the long game, he's strung this out, inflicting misery on tens of thousands of BHS pensioners."

The Pension Protection Fund (PPF) - the lifeboat that picks up pension schemes when companies collapse - also welcomed the move.

The PPF provides protection for those whose pensions are threatened by the BHS collapse, though their retirement incomes under the lifeboat scheme would be lower than they should have been under the original plan.